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/**
 @page dhcpv4Hooks The Hooks API for the DHCPv4 Server

 @section dhcpv4HooksIntroduction Introduction
 BIND10 features an API (the "Hooks" API) that allows user-written code to
 be integrated into BIND 10 and called at specific points in its processing.
 An overview of the API and a tutorial for writing such code can be found in
 the @ref hooksdgDevelopersGuide.  Information for BIND 10 maintainers can be
 found in the @ref hooksComponentDeveloperGuide.

 This manual is more specialised and is aimed at developers of hook
 code for the DHCPv4 server. It describes each hook point, what the callouts
 attached to the hook are able to do, and the arguments passed to the
 callouts.  Each entry in this manual has the following information:

 - Name of the hook point.
 - Arguments for the callout.  As well as the argument name and data type, the
   information includes the direction, which can be one of:
   - @b in - the server passes values to the callout but ignored any data
     returned.
   - @b out - the callout is expected to set this value.
   - <b>in/out</b> - the server passes a value to the callout and uses whatever
     value the callout sends back.  Note that the callout may choose not to
     do any modification, in which case the server will use whatever value
     it sent to the callout.
 - Description of the hook. This explains where in the processing the hook
   is located, the possible actions a callout attached to this hook could take,
   and a description of the data passed to the callouts.
 - Skip flag action: the action taken by the server if a callout chooses to set
    the "skip" flag.

@section dhcpv4HooksHookPoints Hooks in the DHCPv4 Server

The following list is ordered by appearance of specific hook points during
packet processing. Hook points that are not specific to packet processing
(e.g. lease expiration) will be added to the end of this list.

 @subsection dhcpv4HooksPkt4Receive pkt4_receive

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when an incoming DHCPv4
   packet is received and its content is parsed. The sole argument -
   query4 - contains a pointer to an isc::dhcp::Pkt4 object that contains
   all information regarding incoming packet, including its source and
   destination addresses, interface over which it was received, a list
   of all options present within and relay information.  All fields of
   the Pkt4 object can be modified at this time, except data_. (data_
   contains the incoming packet as raw buffer. By the time this hook is
   reached, that information has already parsed and is available though
   other fields in the Pkt4 object.  For this reason, it doesn't make
   sense to modify it.)

 - <b>Skip flag action</b>: If any callout sets the skip flag, the server will
   drop the packet and start processing the next one.  The reason for the drop
   will be logged if logging is set to the appropriate debug level.

@subsection dhcpv4HooksSubnet4Select subnet4_select

 - @b Arguments:
   - name: @b query4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>
   - name: @b subnet4, type: isc::dhcp::Subnet4Ptr, direction: <b>in/out</b>
   - name: @b subnet4collection, type: const isc::dhcp::Subnet4Collection *, direction: <b>in</b>

 - @b Description: this callout is executed when a subnet is being
   selected for the incoming packet. All parameters and addresses
   will be assigned from that subnet. A callout can select a
   different subnet if it wishes so, the list of all subnets currently
   configured being provided as 'subnet4collection'. The list itself must
   not be modified.

 - <b>Skip flag action</b>: If any callout installed on 'subnet4_select'
   sets the skip flag, the server will not select any subnet. Packet processing
   will continue, but will be severely limited (i.e. only global options
   will be assigned).

@subsection dhcpv4HooksLeaseSelect lease4_select

 - @b Arguments:
   - name: @b subnet4, type: isc::dhcp::Subnet4Ptr, direction: <b>in</b>
   - name: @b fake_allocation, type: bool, direction: <b>in</b>
   - name: @b lease4, type: isc::dhcp::Lease4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed after the server engine
   has selected a lease for client's request but before the lease
   has been inserted into the database. Any modifications made to the
   isc::dhcp::Lease4 object will be stored in the lease's record in the
   database. The callout should make sure that any modifications are
   sanity checked as the server will use that data as is with no further
   checking.\n\n The server processes lease requests for DISCOVER and
   REQUEST in a very similar way. The only major difference is that
   for DISCOVER the lease is just selected, but not inserted into
   the database.  It is possible to distinguish between DISCOVER and
   REQUEST by checking value of the fake_allocation flag: a value of true
   means that the lease won't be inserted into the database (DISCOVER),
   a value of false means that it will (REQUEST).

 - <b>Skip flag action</b>: If any callout installed on 'lease4_select'
   sets the skip flag, the server will not assign any lease. Packet
   processing will continue, but client will not get an address.

@subsection dhcpv4HooksPkt4Send pkt4_send

 - @b Arguments:
   - name: @b response4, type: isc::dhcp::Pkt4Ptr, direction: <b>in/out</b>

 - @b Description: this callout is executed when server's response
   is about to be send back to the client. The sole argument - response4 -
   contains a pointer to an isc::dhcp::Pkt4 object that contains the
   packet, with set source and destination addresses, interface over which
   it will be send, list of all options and relay information.  All fields
   of the Pkt4 object can be modified at this time, except bufferOut_.
   (This is scratch space used for constructing the packet after all
   pkt4_send callouts are complete, so any changes to that field will
   be overwritten.)

 - <b>Skip flag action</b>: if any callout sets the skip flag, the server
   will drop this response packet. However, the original request packet
   from a client was processed, so server's state was most likely changed
   (e.g. lease was allocated). Setting this flag merely stops the change
   being communicated to the client.

*/